Hi @god_prophet,
Delivery validation is happening in the campaign (nms:operation) as it appears you are approving the delivery target or content before finally sending the delivery.
The most common error behind this error is that the operator you are using to perform this action has lower privileges than the operator set as the campaign assignee.
If both are the same operator, or the approving operator has higher privileges (e.g. admin) then delivery validation should work as expected, unless there is no other backend failure.
BR,
Ishan